It is known that an ethylene/vinyl acetate copolymer (EVA) having a vinyl acetate content of 0.5 to 30 percent by weight has both thermoplastic and elastomeric properties and that EVA having a vinyl acetate content of 30 to 80 percent by weight has rubber-like properties. The former is used, for example, in extrusion molding, blow molding, injection molding, calendering, and foaming while the latter is used, for example, in synthetic rubber, in grafting or blending, as a plastic modifier, and in hot-melt adhesives.
As the vinyl acetate content increases, the flexibility, especially the low temperature flexibility, and the loading of inorganic fillers are both improved. On the other hand, the strength and heat resistance deteriorate with the result that the melt index becomes very high. This increases the tackiness and blocking, which, in turn, lowers the workability of the copolymer and the mold and roll releasability. Further, the copolymer pellets or powder tend to mass.
Thus, the EVA with a vinyl acetate content of 30 percent by weight or higher is unsuitable for extrusion and is unusable in extrusion molding.
To counter this disadvantage, organic peroxides have been added to the EVA with high vinyl acetate content. The EVA is then heat kneaded to lower the melt index. Unfortunately, this leads to the formation of high molecular weight components, which results in film of poor appearance because of high gel levels.